The present disclosure is directed to semiconductor memory devices, and more particularly, to magnetic memory devices.
In general, a magnetic random access memory (MRAM), which is a nonvolatile memory device, stores data by using a magnetoresistance effect in which the resistance of an electrical conductor varies according to a peripheral magnetic field. The MRAM may include a plurality of MRAM cells including a magnetic tunnel junction (MTJ).
An MTJ may be configured to include a sandwich-type multi-layer thin film in which electrons can tunnel through a very thin dielectric layer disposed between two ferromagnetic thin layers when an external electrical signal is applied thereto. One of the two ferromagnetic thin layers is called a free layer, and the other is called a pinned layer.
When the magnetization directions in the free layer and the pinned layer are arranged in parallel to each other, a tunneling current flowing through the MTJ becomes maximized. That is, a tunneling resistance is minimized. On the other hand, when the magnetization directions in the free layer and the pinned layer are arranged in anti-parallel to each other, a tunneling current flowing through the MTJ becomes minimized. That is, a tunneling resistance is maximized.
A typical memory uses an electric charge to store data, whereas the MRAM uses a magnetic charge to store data. That is, digital data are represented by differing polarities of the ferromagnetic layers, in which a ‘0’ is represented by a low-resistance state where the magnetization directions of the two magnetic layers are parallel to each other and a ‘1’ is represented by a high-resistance state where the magnetization directions of the two magnetic layers are anti-parallel to each other. Data may be written to memory cells by passing a current through a pair of perpendicular wires above and below the cell, which creates an induced magnetic field at the junction, which is picked up by the free, writable layer. However, as a cell is scaled down in size, the induced field will overlap adjacent cells over a small area, leading to potential false writes, a phenomenon known as write disturbance.
Recently, MRAM devices suitable for a spin injection mechanism have been proposed to overcome write disturbance and low integration density.
However, a write current density must be higher than a critical current density to switch a selected MRAM cell by using the spin injection mechanism. This increases the power consumption of an MRAM, which otherwise would restrict the increased integration density of the MRAMs.